> A sunk cost fallacy is the idea that because you've invested a certain amount of resources (time, in this case) in a particular approach, it would be foolish

> A sunk cost fallacy is the idea that because you've invested a certain amount of resources (time, in this case) in a particular approach, it would be foolish to switch because all the resources you've already expended would be "wasted." No, you're using the wrong word here. It isn't a sunk cost when you've invested resources (i.e. can recover value from those resources in the future), it's when you've spent (i.e. cannot recover value from the resources) resources. The knowledge gained from learning Rails is obviously recoverable, and can be directly applied to future work. Throwing that away is not a sunk cost issue.